A method is described in German Patent Application No. DE 198 54 306. In this patent application, a fuel injection device is described, which includes a piezoelectric actuator and which is installed in an internal combustion engine having gasoline direct injection (GDI). In an internal combustion engine of this type, a separate fuel injection device is assigned to each combustion chamber. It is possible to inject the fuel into the corresponding combustion chamber of the internal combustion engine via the fuel injection device in such a way that the fuel is distributed in the combustion chamber as a function of the desired operating mode, stratified or homogeneous. If it is stratified, this means that an ignitable fuel-air mixture is essentially present only in the area of the spark plug. In this operating mode, the air feed to the combustion chamber is essentially completely unthrottled.
The use of piezoelectric actuators has the advantage that they make it possible to inject a desired fuel quantity with high precision. This applies in particular to very small injected fuel quantities, such as occur, for example, in a pilot injection or when larger injected fuel quantities are broken down into small single injections.
In order to inject fuel into a combustion chamber, an electrical charge is introduced into the piezoelectric actuator. This charge results in a change in the length of the piezoelectric actuator. A valve element of the fuel injection device and the piezoelectric actuator are normally coupled together via hydraulic amplification. To end the injection, the charge is again drained off the piezoelectric actuator. The piezoelectric actuators of the fuel injection devices of the particular combustion chambers or cylinders of an internal combustion engine are normally triggered by triggering an output stage located in a control unit. The control unit and the piezoelectric actuator are connected via leads which are detachably connected to the fuel injection device using connecting elements.
In a system of this type, it may occur that the piezoelectric actuator is triggered in such a way that the fuel injection device is opened and, due to a malfunction, it is simultaneously no longer possible to change the charge state of the piezoelectric actuator and thus to close the fuel injection device. As a consequence, fuel would be supplied to the corresponding cylinder without interruption, i.e., both during the combustion cycle and during the ejection of the hot exhaust gases, for example. However, this would result in serious damage to the internal combustion engine and to a catalytic converter of the internal combustion engine caused, for example, by a fuel knock.
Such an error may occur, for example, in the event of a cable rupture in the connection between the piezoelectric actuator and the control unit. Furthermore, this error may also occur if a defect occurs in the control unit. A defect of this type is present, for example, in a defective output stage switch. Further, an error of this type occurs when the plug connection on the control unit and/or on the piezoelectric actuator has become detached.
Such errors may be detected by the control unit. German Patent Application No. DE 198 54 306 describes an ohmic resistor connected in parallel to the piezoelectric actuator, via which the piezoelectric actuator may be discharged in a defined manner. Despite this measure, it has been determined, however, that it is not possible to rule out with absolute certainty that an amount of fuel such as to cause a fuel knock there will still enter the combustion chamber of the internal combustion engine. It is also not possible to completely rule out damage to the catalytic converter due to fuel entering the combustion chamber during the exhaust stroke, for example.